Foundation structure of prestressed concrete tank

ABSTRACT

A foundation structure of a prestressed concrete tank that is constructed on a hard supporting ground and that has a structure in which a base slab and a side wall are integrally formed, comprises a flexible layer that is provided on the supporting ground and that maintains flexibility even when the flexible layer receives the weight of the prestressed concrete tank. The prestressed concrete tank is constructed on the flexible layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority upon Japanese Patent Application No. 2003-308328 filed Sep. 1, 2003, the contents of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to foundation structures of prestressed concrete tanks.

2. Description of the Related Art

Prestressed concrete (PC) tanks having a structure in which the base slab and the side wall are integrally formed are known as cylindrical tanks for storing, for example, liquefied natural gas (LNG), liquefied petroleum gas (LPG), service water, or wastewater. Such PC tanks are often constructed on soft ground in metropolitan areas according to pile-foundation methods. If PC tanks gain popularity in rural areas, then they would be constructed on hard ground, such as bedrocks, according to spread-foundation methods.

If, however, a PC tank is constructed on hard supporting ground according to spread-foundation methods, then displacement (or movement) of the base slab of the PC tank will be constrained by the hard supporting ground. As a result, when the PC tank is deformed due to, for example, the prestress that is applied to the PC tank in the circumferential direction, the inner pressure of the liquefied gases stored therein, or changes in temperature, a large bending moment will be generated at the lower edge of the side wall and thus stress will concentrate thereon. This may lead to cracks in the tank. The lower edge of the side wall is an important section in terms of the functions and performance of the PC tank, and therefore, no cracks should be present in that section. In order to cancel the negative influence caused by the bending moment described above, it is necessary to take countermeasures such as applying a large prestress, in the vertical direction, to the side wall of the PC tank, or increasing the sectional thickness of the lower edge of the side wall. Such countermeasures, however, lead to a significant increase in costs and complication in structure.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above and other problems, and an object thereof is to provide a countermeasure that allows the stress that is generated at the lower edge of the side wall of the tank to be reduced at low costs for cases where the PC tank, which is structured such that the base slab and the side wall are integrally formed, is to be constructed on hard supporting ground.

An aspect of the present invention is a foundation structure of a prestressed concrete tank that is constructed on a hard supporting ground and that has a structure in which a base slab and a side wall are integrally formed, the foundation structure comprising:

-   -   a flexible layer that is provided on the supporting ground and         that maintains flexibility even when the flexible layer receives         the weight of the prestressed concrete tank,     -   wherein the prestressed concrete tank is constructed on the         flexible layer.

Features and objects of the present invention other than the above will become clear by reading the description of the present specification with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate further understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a plan view of a PC tank according to an embodiment of the present invention;

FIG. 2 is a cross section of the PC tank according the embodiment taken along line II-II of FIG. 1;

FIG. 3 is a cross section, also taken along line II-II of FIG. 1, showing an enlargement of a side wall of the PC tank according to the embodiment;

FIG. 4 is an enlarged cross section showing a structure of a flexible layer according to the embodiment;

FIG. 5 is a diagram for illustrating actions of a bag that structures the flexible layer;

FIG. 6 is a diagram showing an example of a deformed state of the side wall of the PC tank according to the embodiment;

FIG. 7 is a diagram showing the bending moment that is generated at the side wall in the deformed state shown in FIG. 6; and

FIG. 8 is a diagram showing the circumferential force that is generated at the side wall in the deformed state shown in FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

An aspect of the present invention is a foundation structure of a prestressed concrete tank that is constructed on a hard supporting ground and that has a structure in which a base slab and a side wall are integrally formed, the foundation structure comprising:

-   -   a flexible layer that is provided on the supporting ground and         that maintains flexibility even when the flexible layer receives         the weight of the prestressed concrete tank,     -   wherein the prestressed concrete tank is constructed on the         flexible layer.

According to the present invention, the PC tank is constructed on a flexible layer, and therefore, even when the PC tank is deformed due to, for example, the prestress that is applied to the PC tank in the circumferential direction, the inner pressure of the liquefied gases stored therein, or changes in temperature, it is possible for the section that connects the side wall and the base slab of the PC tank to move (i.e., be displaced) in accordance with the deformation. In this way, it becomes possible to reduce the stress generated at the lower edge of the side wall. Further, it is also possible to achieve reduction in costs because it is only necessary to provide a flexible layer on the supporting ground.

Further, the flexible layer may be structured by laying bags on the supporting ground to cover the ground, each of the bags being filled with a powder substance. In this way, it is possible to achieve further reduction in construction costs because the flexible layer can be structured simply by filling inexpensive earth-and-sand into inexpensive bags and laying those bags on the supporting ground to cover the ground.

According to the present invention, it becomes possible to reduce the stress that is generated at the lower edge of the side wall of the tank at low costs for cases where the PC tank, which is structured such that the base slab and the side wall are integrally formed, is to be constructed on hard supporting ground.

FIG. 1 is a plan view of a PC tank 10 according to an embodiment of the present invention. FIG. 2 is a cross section of the PC tank 10 taken along line II-II of FIG. 1. FIG. 3 is a cross section, also taken along line II-II of FIG. 1, showing an enlargement of a side wall of the PC tank 10. The PC tank 10 is for storing, for example, LNG or LPG.

The PC tank 10 is made of concrete in which a base slab 14 and a side wall 16 are formed integrally. The PC tank 10 is constructed on hard supporting ground 12. Prestress is applied to the PC tank 10 in both the circumferential and vertical directions by prestressing steel, which are not shown. The prestress enables the tank to endure the pressure of the liquid (e.g., LNG or LPG) stored in the PC tank 10.

As shown in FIG. 2 and FIG. 3, the base slab 14 of the PC tank 10 is constructed on a flexible layer 20 provided on the supporting ground 12. The base slab 14 is constructed on the flexible layer 20 in this way in order to avoid the conventional problems such as those described in the section of “Description of the Related Art”.

More specifically, if the PC tank 10 is constructed on the hard supporting ground 12 according to spread-foundation methods, then the base slab 14 will be constrained by the supporting ground 12. As a result, when the PC tank 10 is deformed due to, for example, the prestress that is applied to the PC tank in the circumferential direction, the inner pressure of the contents stored therein, or changes in temperature, the section (i.e., the lower edge of the side wall 16; referred to below as “side-wall base section 17”) that connects the base slab 14 and the side wall 16, which are formed integrally, cannot be displaced. Thus, a large bending moment is generated at the side-wall base section 17. The side-wall base section 17 is the most important section in terms of the functions and performance of the PC tank 10, and therefore, it is not preferable for a large bending moment to be generated at this section because this may cause cracks due to stress concentration. Conventional countermeasures for canceling the negative influence caused by the bending moment involve applying a large prestress also in the vertical direction, or increasing the sectional thickness of the side-wall base section. Such countermeasures, however, lead to a significant increase in costs and an increase in time and labor for construction.

In view of the above, in the present embodiment, the PC tank 10 is constructed on a flexible layer 20. In this way, it is possible for the side-wall base section 17 to move (i.e., be displaced) in accordance with the deformation of the PC tank 10 caused by the above-described prestress, inner pressure, temperature change, etc., and thus, it is possible to reduce the bending moment generated at this section.

FIG. 4 is an enlarged cross section showing the structure of the flexible layer 20 according to the present embodiment. As shown in FIG. 4, the flexible layer 20 is structured by laying, on the supporting ground 12 such as to cover the ground 12, bags 22 made of cloth and into which a powder substance, such as earth-and-sand, is filled. Oversite concrete 24 is provided on the flexible layer 20. Reinforcing bars of the base slab 14 of the PC tank 10 are arranged on the oversite concrete 24, and concrete is poured thereover.

As shown in FIG. 5, according to the structure of the flexible layer 20 described above, when a downward load F is applied from the PC tank 10 to the flexible layer 20, the earth-and-sand in the bag 22 move in such a direction as to deform the bag 22 so that it becomes flat, whereby a film tension T is caused in the bag 22. This causes a force P in such a direction as to return the earth-and-sand to their original positions, and thus, flexibility and restorability of the flexible layer 20 in the vertical direction can be achieved. Thus, the earth-and-sand in the bag 22 will not harden nor be compressed by the load of the PC tank. Therefore, even when a load from the PC tank 10 is applied after construction of the tank, the flexibility of the flexible layer 20 is maintained. Thus, even when the PC tank 10 is deformed due to the above-described prestress, inner pressure, temperature change, etc., it is possible for the side-wall base section 17 to move (i.e., be displaced) in accordance with the deformation. Consequently, it is possible to reduce the bending moment generated in this section.

FIG. 6 is a diagram showing an example of a state in which the side wall 16 of the PC tank 10 has deformed. The up/down direction in the figure corresponds to the vertical direction of the side wall 16. FIG. 7 and FIG. 8 respectively show the bending moment and the circumferential force that are generated at the side wall 16 in the deformed state shown in FIG. 6. It should be noted that in FIG. 6 through FIG. 8, the solid line is indicative of an example in which the PC tank 10 is constructed on the flexible layer 20 as in the present embodiment, and the broken line is indicative of an example in which the PC tank 10 is constructed directly on the hard supporting ground 12.

As shown by the solid line in FIG. 6, in the present embodiment, construction of the PC tank 10 on the flexible layer 20 allows the section connecting the base slab 14 and the side wall 16 to move such that it sinks downwards while rotating. Correspondingly, as shown in FIG. 7, the intensity of the bending moment at the side-wall base section 17 is reduced. As a result, it is possible to achieve a reduction in the amount of prestress that is to be applied to the side wall 16 in the vertical direction. Therefore, cost reduction, which reflects the reduction in prestress, can be achieved. Further, since the side-wall base section 17 can move, it is possible to apply a compressive force σs, in the circumferential direction, also to the side-wall base section 17 as shown in FIG. 8, and thus, a prestressing effect can be achieved.

As described above, according to the present embodiment, it is possible to reduce the bending moment at the side-wall base section 17 by constructing the PC tank 10 on the flexible layer 20 provided on the supporting ground 12. Therefore, it is not necessary to increase the sectional thickness of the side wall 16 as with conventional countermeasures, and also, it is possible to reduce the amount of prestress that is to be applied in the vertical direction for canceling the tensile stress in the side-wall base section 17. Consequently, the PC tank can be designed rationally, and thus, cost reduction can be achieved.

Further, it is possible to use inexpensive sandbags as the bags 22 for structuring the flexible layer 20. Further, materials such as earth-and-sand that are obtainable on site can be used as the material for filling the bags. Since the flexible layer 20 can be structured simply by laying those inexpensive bags 22 on the supporting ground 12 such as to cover the ground 12, it is possible to keep construction costs extremely low.

The material for structuring the flexible layer 20, however, is not limited to bags 22 filled with earth-and-sand. Any kind of material, such as Styrofoam, can be used for structuring the flexible layer 20 as long as it does not lose its flexibility even in a state where the heavy load from the PC tank 10 is applied thereto.

Although a preferred embodiment of the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from spirit and scope of the inventions as defined by the appended claims. 

1. A foundation structure of a prestressed concrete tank that is constructed on a hard supporting ground and that has a structure in which a base slab and a side wall are integrally formed, said foundation structure comprising: a flexible layer that is provided on said supporting ground and that maintains flexibility even when said flexible layer receives the weight of said prestressed concrete tank, wherein said prestressed concrete tank is constructed on said flexible layer.
 2. A foundation structure of a prestressed concrete tank according to claim 1, wherein said flexible layer is structured by laying bags on said supporting ground to cover said ground, each of said bags being filled with a powder substance.
 3. A foundation structure of a prestressed concrete tank according to claim 2, wherein said powder substance is earth-and-sand.
 4. A foundation structure of a prestressed concrete tank according to claim 2, wherein said bags are made of cloth.
 5. A foundation structure of a prestressed concrete tank according to claim 3, wherein said bags are made of cloth.
 6. A foundation structure of a prestressed concrete tank according to claim 1, wherein: oversite concrete is provided on said flexible layer; and said base slab of said prestressed concrete tank is constructed on said oversite concrete.
 7. A foundation structure of a prestressed concrete tank according to claim 2, wherein: oversite concrete is provided on said flexible layer; and said base slab of said prestressed concrete tank is constructed on said oversite concrete. 